1 Silica Gel Quantity Recommendation
How Much Arten Gel or Rhapid Gel is required to condition a standard exhibition case or sealed storage unit?
TEMPORARY EXHIBITS: 1 SACHET (300 GRAMS) PER 3.0 CUBIC FOOT
PERMANENT EXHIBITS: 1 SACHET (300 GRAMS) PER 1.5 CUBIC FOOT
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These recommendations, based on an MH value of 5.0, apply to both Rhapid Gel and Arten Gel
Less Rhapid Gel or Arten Gel can be used for well-sealed cases or for shorter periods on exhibition. Moderate external RH conditions and a large acceptable RH range within the sealed volume will also reduce the amount of gel required.
The recommended quantity of silica gel is determined based on an equation that can be used for any type of silica gel. The calculation incorporates key factors including the silica gel’s RH buffering capacity, case leakage rate, the duration of exhibition, and internal/external RH conditions.
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Since some of the factors such as case leakage and external RH conditions are not known, Art Preservation Services has developed a “generic” recommendation that is based upon standard museum practices and experience.
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The recommendation for Arten Gel and Rhapid Gel assumes a half-time air exchange rate of one day and a temporary exhibition duration of 90 days. An MH value of 5.0 is used for Arten Gel (below 40% RH) and Rhapid Gel (within the range of 0-60%).
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Comparing Manufacturer Recommendations for How Much
Silica Gel is Required: What is the Correct Quantity?
The amount of silica gel required to condition a case varies from manufacturer to manufacturer. This has resulted in conflicting recommendations regarding the correct amount of silica gel to use for a specific application. Unfortunately, most manufacturers do not provide information on how they arrive at their recommendation for how much silica gel is required.
Regardless of what type of calculation is used, the key variable that differentiates one silica gel from another is the silica gel’s buffering capacity, measured by its MH Value.
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Comparing Relative Quantity for Different Types of
Silica Gels to Achieve Equivalent Performance
A higher MH value indicates better performance. The bar graph below shows that a higher MH value equates to a lower amount of silica gel required for comparable RH buffering performance (Q).
The accompanying bar graph compares the relative amount of silica gel required for equivalent RH buffering performance.
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The Relative Quantity Required (Q) is calculated by dividing the MH value of each gel by a reference MH value.
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For this graph, the reference gel is Rhapid Gel at 40-60% (MH=5.4).
This graph illustrates the importance of accounting for a silica gel’s MH Value to determine the correct quantity of silica gel required for a specific application.
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Comparing Silica Gel Quantity Recommendations: An Example
As an example, one of the museum-grade gels, Art Sorb, recommends around 29 grams/cubic foot. Art Preservation Services recommends 100 grams/cubic foot, based on “typical” case performance. Does this mean that Art Sorb is about 300% more efficient than Rhapid Gel or Arten Gel because so little Art Sorb is required compared to other types of silica gel?
Equivalent Performance (Grams/Cubic Foot)
RH Range
20-30%
40-60%
Arten Gel
100g
Not
Recommended
Rhapid Gel
120g
100g
Art Sorb
190g
140g
According to the Equivalent Performance table, significantly less Rhapid Gel or Arten Gel is required compared to Art Sorb within the range of 0-60% RH.
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Within the range of 20-30% RH, Arten Gel is 90% more efficient than Art Sorb.
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Within the range of 40-60% RH, Rhapid Gel is 40% more efficient than Art Sorb.
This example illustrates why a vendor’s Quantity Recommendation must provide information on the gel’s MH value and on how the recommendation was determined to assess the gel’s relative efficiency.
Comparing the Cost-Effectiveness of Different Types of Silica Gel
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The relative cost-effectiveness of a museum-grade silica gel is based on the gel’s MH value and the dry weight cost of the gel. A manufacturer may provide a recommendation for how much silica gel to use that underestimates the amount that is required. The gel may appear to be more cost-effective than other gels that use a more realistic quantity recommendation.
As a hypothetical example, within the range of 40-60% RH, the manufacturer of Gel X (MH = 3.0) recommends 25 grams/cubic foot and the manufacturer of Gel Y (MH = 4.5) recommends 100 grams/cubic foot.
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Gel X costs $10.00/100 grams. Based on the recommendation of 25 grams/cubic foot, the cost would be $2.50/cubic foot.
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Gel Y costs $3.75/100 grams. Based on the recommendation of 100 grams/cubic foot, the cost would be $3.75/cubic foot.
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Because of the lower quantity recommendation for Gel X, it appears that Gel X is less expensive per cubic foot, even though it is 2.5 times more expensive than Gel Y based on weight. This is an incorrect assumption.
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A higher quantity of Gel X would be required relative to Gel Y for comparable performance since Gel X has a lower MH value. Even if both gels had the same cost per cubic foot, Gel Y would be the more cost-effective option since less gel would be required.
Regardless of the formula used to determine the amount of silica gel required per cubic foot, the gel’s MH values must be considered when calculating quantity and cost-effectiveness.
The method for comparing cost-effectiveness is to multiply the relative quantity required for equivalent performance (Q) by the relative unit cost per unit weight ($/grams):
relative cost - effectiveness = Q x $/grams
In this example, a Q value of 1.0 is used for Gel Y (MH=4.5). Since Gel X (MH=3.0) would require 1.5 times more to provide equivalent performance, Gel X's Q Value is 1.5.
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The cost of Gel X is $10.00/100 grams.
The relative cost of equivalent performance of 100 grams is $15.00
(1.5 x $10.00)
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The cost of Gel Y is $3.75/100 grams.
The relative cost of equivalent performance of 100 grams is $3.75
(1.0 x $3.75)
Based on these numbers, Gel X is 4 times as expensive as Gel Y based on the amount required per cubic foot.
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Some websites provide a silica gel calculator that allows the user to enter data about case volume, case leakage, time on exhibition and overall RH conditions. However, if the calculator does not account for the correct MH based on the RH range of use, it will not provide accurate results.
For example, if the calculator is based on an A-Type gel, the MH value can vary between 1.5 and 6.0 within the range of 20-60% RH.
If a constant MH value of 6.0 is used over the entire RH range, the calculator will underestimate the amount of A-Type silica gel required within the 50-60% range by a factor of 4, since the actual MH value is only 1.5.
Utilizing a Website Silica Gel Calculator to Determine Quantity Required
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The MH values used in the Moisture Buffering Capacity table and throughout this website are derived from two sources.
Canadian Conservation Institute’s Technical Bulletin 33 on Silica Gel
Unpublished data based on hysteresis studies done by Art Preservation Services.
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